Mechanically planar chiral [2]rotaxanes were synthesized by the introduction of bulky pyrrole moieties into the axle component of an achiral [2]rotaxane. The enantiomers were separated by chiral HPLC. The shuttling of the ring component between the two compartments at high temperature induced the stereoinversion of the mechanically planar chiral [2]rotaxane. The rate of the stereoinversion was studied quantitatively, and the kinetic parameters were determined.
An effective route to multisubstituted cyclohexanes has been developed by exploiting the DielsAlder reaction of easily available dienes within a dioxanone moiety with electrondeficient dienophiles.Modular assembly of organic molecules with sizable molecular weight and/or complexity is gaining increasing importance for developing effective routes to architecturally complex, biologically active natural/unnatural products.1 Associated with the importance of cyclohexane motifs with functional and stereochemical complexity, 2 we report herein assembly of a cyclohexane skeleton from three components, that is, an olefin, an aldehyde, and a ¢-keto ester equivalent, that is, iododioxinone I (Scheme 1).As the key conjunctive agent of our plan for assembling three components, we focused on diene II as the synthetic platform, which could be derived from iododioxinone I and an aldehyde (Scheme 2). Three promising features in II are, (1) high DielsAlder reactivity expected from two vicinal exocyclic alkenes (s-cis diene), (2) characteristic reactivity by donor/ acceptor substitution pattern, 3 and (3) capability of generating acyl ketene species from the dioxinone moiety 4 that is regenerated by the DielsAlder reaction, amenable for various synthetic manipulations. In addition, the dioxanone scaffold may also provide a platform for stereoselective reactions. 5 We report herein facile synthesis of dienes II and their excellent behaviors in the DielsAlder reactions.Readily available iododioxinone 1 6,7 served as a platform to various dienes. The protocol is exemplified by the preparation of diene 3a (Scheme 3). To a mixture of iodide 1 and cyclohexanecarbaldehyde in Et 2 O (¹90°C) was added n-BuLi, where a rapid halogenlithium exchange followed by the carbonyl addition gave alcohol 2a in 91% yield.8 Alcohol 2a, thus obtained, was treated with triflic anhydride in the presence of Hünig's base, giving cleanly diene 3a as the single product in 80% yield. The (Z,Z) stereochemistry was proven by NOE study.Scheme 4 shows a rationale for the (Z,Z)-selectivity; Given the 1,4-elimination occurred from the intermediary triflate with the anti and/or syn relationship of the proton and the triflate, 9 the 1,3-allylic strain 10 suggests that the reaction would occur from conformers A and/or B, either of which gives the (Z,Z) isomer.This protocol allowed facile access to several other dienes 3b3e from the corresponding aldehydes (Scheme 5). Rigorous (Z,Z)-selectivity applied in all of these cases. A limitation was that the reaction of alcohol 2b, the propanal adduct, produced also a positional isomer 4b in 38% yield.Having these dienes in hand, we examined their reactivity in the DielsAlder reactions 7 (Table 1). Upon reaction of 3a with N-phenylmaleimide (toluene, room temp., 1 h), the endo cycloadduct 5a was obtained as a single diastereomer in 95% yield (Run 1). The stereochemistry of 5a was confirmed by X-ray analysis 11 (Figure 1). Other dienes were also subjected to the reaction with N-phenylmaleimide, giving good to excellent yield of ...
Expeditious Modular Assembly of Multisubstituted Cyclohexanes via Dioxanone-Dienes. -An effective one-pot method for the title compounds (III) and (VII) is presented from the easily available alcohols (I). In all reactions a single diastereomer is obtained. The cycloadduct (IIIa) can be transformed to the compound (V) for example. -(AOKI, Y.; MOCHIZUKI, Y.; YOSHINARI, T.; OHMORI, K.; SUZUKI*, K.; Chem. Lett. 40 (2011) 10, 1192-1194, http://dx.
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